In the use of an air knife as the doctoring means, the coating mix applied to the web is smoothed by blowing air against the web surface at a high velocity from a narrow orifice of the air knife toward the web. The air jet cuts away or removes excess coating from the web surface in the form of an atomized spray, and the emitted coating mist is collected in a special chamber and recycled back to the coating mix pool. The use of the air knife results in a constant-thickness coat, and the profile of the coated paper or board conforms to the base web profile. The covering power of the applied coat is high.
In air-knife coating, excess coating is doctored away with the help of a sharp air jet discharged from a narrow nozzle orifice. The velocity of the air jet may be as high as 0.7-0.8 Mach. Typically the pressure of the air jet is 0.2-1 bar. The principal reason for using air-knife coating is that this method achieves a coat with an extremely constant thickness and high covering power owing to the good conformance of this contour-type coat to the base paper or board surface profile. An essential factor in achieving such a contour-type coat is filter cake formation. The filter-cake layer is formed when the water and the binding agents of the coating mix are absorbed from the fluid coating mix by the base paper. The filter-cake layer is formed along the contour of the paper or board sheet surface. Then, the air jet can cut off practically all excess fluid coating down to the filter-cake layer.
Conventionally, the control of coat weight in air-knife coating has been attempted by means of adjusting the air-knife pressure. Herein, the control facilities are rather limited, because when a light coat weight is desired, also a portion of the top surface of the filter-cake layer should be removed, which requires use of high-power air jets and causes a number of problems such as fuming. on the other hand, when heavy coat weights are desired, stability problems arise in the thickness control of the coating layer, because the air jet must perform the cutting of excess coating in a fluid-state coating layer lacking a well-defined phase boundary.
In air-knife coating, application and doctoring of the coat form two, clearly separate steps. During the application step the coating mix is metered onto the web and the applied amount of coating is larger than the desired final coat weight. The excess coating is cut off or removed with the help of an air jet discharged from a narrow orifice. Between the application and the doctoring steps, interaction occurs between the paper sheet and the coating mix, whereby water and binding agents are absorbed to the base sheet chiefly from the layers of coating closest to the web surface. This absorption of water and binding agents is called penetration, which phenomenon involves an increase in the solids content of the layers of coating closest to the web surface. This increase in solids content results in filter cake formation, which means that onto the base sheet is deposited a coating layer whose solids content is high enough to cause settling of the coating mix, whereby the coating ceases to flow.
Usually the coarse control of coat weight is implemented by adjusting the doctoring conditions so that doctoring is performed based on the filter-cake phenomenon. The fine control of coat weight is then accomplished by adjusting the air jet pressure. However, in practice the air jet is capable of cutting off only a limited amount of coating as, when a light coat weight is desired, the required air jet power increases dramatically. Use of a high-power jet results in deleterious fuming of the coating and increased noise emission from the air knife. The apparatus also needs effective compressed-air generators and as the air blown from the orifice must be oil-free and clean, the total costs of the apparatus rise rapidly with the increase in the required doctoring effect and resulting elevated air demand. By contrast, at heavy coat weights the air-knife coating method fails to give a smooth coating as the low-viscosity, low-solids coating mix portion to be doctored detaches irregularly from the web surface and the doctoring action becomes extremely unstable to control. By modifying the properties of the coating mix, the thickness of the filter-cake layer formed on the web surface between the application step and the doctoring step can be affected. Doctoring succeeds best along the upper surface of this settled layer, whereby also the doctoring step behaves in a stable manner. However, the control of the coating mix properties is extremely clumsy and difficult to manage in a controlled manner. The preparation of different coating mix formulas is time-consuming, which makes run-time thickness control of the filter-cake layer of the applied coating impossible in practice. Hence, this approach can be used only for minor adjustment of the coating process conditions.